Conventionally, a power feeding assembly (lighting device) for feeding a power to a light emitting diode (LED) illumination module has been provided, which is disclosed, e.g., in Japanese Patent Application Publication No. 2006-511078 (JP2006-511078A). As shown in FIG. 17, the prior art example described in JP2006-511078A includes a series circuit of a diode D10 and a control switch 101 configured with a MOSFET which are connected to both ends of a DC power supply 100. In addition, an inductor L10 and an LED illumination module 102 are connected to both ends of the diode D10. A controller 103 generates a dual-PWM (Pulse-Width Modulation) switching signal supplied to a control input unit of a control switch 101 through an amplifier 104. The dual-PWM switching signal is a combination of a high-frequency PWM switching signal component and pulse bursts of a low-frequency, i.e., a low-frequency PWM switching signal component.
The controller 103 includes a current mode pulse width modulator 105, which receives an LED current reference signal, a detection current, and a high-frequency sawtooth wave signal from a current supply 106. The current mode pulse width modulator 105 generates a high-frequency PWM switching signal component supplied as one input of an AND gate 107, and the other input of the AND gate 107 is a low-frequency PWM switching signal component. An output from the AND gate 107 is supplied to a gate of the control switch 101 through the amplifier 104.
Thus, in the prior art example, an average current flowing through the LED illumination module 102 can be changed by changing the low-frequency component of the dual-PWM switching signal, and thus, the intensity of light output from the LED illumination module 102 is changed.
However, in the prior art example disclosed in JP2006-511078A, the dual-PWM switching signal supplied to the control input unit of the control switch 101 (switching element) is an AND output of the low-frequency PWM signal and the high-frequency driving signal. For this reason, as shown in FIG. 18A, when the PWM signal falls during an ON period of the control switch 101, the driving signal from the control switch 101 becomes a low level. In this manner, the ON period of the control switch 101 is changed depending on the change in the ON duty ratio of the PWM signal, and accordingly, a load current flowing through the LED illumination module 102 (light source unit), i.e., a light output from the LED illumination module 102, changed. Thus, dimming of the LED illumination module 102 is performed by changing the ON duty ratio of the PWM signal. Also, the waveform shown in FIG. 18A is an example when the control switch 101 is operated in a critical current mode.
Meanwhile, during an OFF period of the control switch 101, since a flyback current of the inductor L10 flows to the LED illumination module 102 through the diode D10, although the PWM signal falls during the corresponding period, a light output from the LED illumination module 102 is not changed. That is, as shown in FIG. 18A, within the range indicated by the dashed single-dotted line in the same drawing, although the ON duty ratio of the PWM signal is swept, a subsequent ON pulse of the driving signal of the control switch 101 is not generated. For this reason, during the interval indicated by the arrow in FIG. 18A, although the ON duty ratio of the PWM signal is swept, the light output from the LED illumination module 102 is not changed. Thus, as shown in FIG. 18B, with respect to the ON duty ratio of the PWM signal, the light output from the LED illumination module 102 is changed stepwise. A light output difference by one step is equivalent to a light output of one cycle of the driving signal of the control switch 101.
Thus, in the prior art example described in JP2006-511078A, when the PWM signal is swept, the light output from the LED illumination module 102 is changed by one step at a time, causing problems in which the light output is not changed smoothly so that a user can see the notable change. In particular, in the prior art, when the LED illumination module 102 is dimmed at a low luminous flux, the change ratio of the light output from the LED illumination module 102 is increased, and thus, the change is seen further notable.
Further, when the LED illumination module 102 is imaged through various imaging devices such as a video camera or the like, the frequency of the PWM signal is required to be increased to have a certain value or higher to prevent blinking due to an interference with a frequency of the imaging device from being observed. In this case, however, when the frequency of the PWM signal is increased, the ratio of one period of the driving signal of control switch 101 to one period of the PWM signal is increased. Then, the light output is increased by one period of the driving signal of the control switch 101 and it is more conspicuously seen such that the light output from the LED illumination module 102 is changed by one step at a time.
In order to avoid this, the frequency of the driving signal of the control switch 101 is required to be increased, but considering an increase in a switching loss or an upper limit of the frequency of the driving signal in case of driving with a low-priced part such as a general IC, and the like, a desirable high-frequency is hardly guaranteed.